Field of the Invention
The present invention relates to novel corrosion inhibitor compositions for magnesium or magnesium alloys and to a process for inhibiting the corrosion of such metals using such compositions.
Brief Description of Related Technology
Magnesium is most lightweight of all structural metals, weighing 35 percent less than aluminum and 78 percent less than steel. Lightweight characteristics, wide availability and processability make magnesium alloys suitable for production of motor vehicle components, electric products, aircraft components, etc. Generally, magnesium and magnesium alloys are formed into shaped articles by die casting, extrusion or rolling. However, the percentage of magnesium alloys used in automobiles, electric products, aircraft components, etc. has been traditionally low. The reasons for the limited use of magnesium alloys are associated with the intrinsic properties of this family of alloys: low creep and corrosion resistance.
The corrosion resistance of magnesium or magnesium alloys depends on similar factors that are critical to other metals. However, because of the electrochemical activity of magnesium, the relative importance of some factors is greatly amplified. When unalloyed magnesium is exposed to air at room temperature, a gray oxide forms on its surface. Moisture converts this oxide to magnesium hydroxide, which is stable in the basic range of pH values, but not in the neutral or acid ranges.
For providing anti-corrosion properties, magnesium or magnesium alloys are generally treated with chromates. The chromate treatment nevertheless involves the difficulty in setting the conditions for the treatment, so that it has been desired to provide more convenient corrosion inhibiting processes. Furthermore, the chromate treatment has the drawback that when conducted, the treatment discolors the surface of the metal, depriving the metal of its luster. Furthermore, chromium compounds are rather toxic and harmful to the environment. Thus, processes are highly desirable which are less likely to burden the environment.
For achieving corrosion protection, magnesium can also be coated in an assortment of ways depending on the type of alloy used, the desired qualities of the finished material and the application in which it will be used. For example, magnesium can be coated with organic layers. These layers prevent against corrosion of the magnesium by insulating it from the outside environment.
Advanced coating systems also possess active corrosion protection that implies continuous corrosion protection, even in the event of local damage to the coating. This is achieved by incorporating corrosion inhibitors in various components of multilayer coating system: in conversion coating, pre-treatment or even in the top coat.
For many years, chromates have been used as the most effective corrosion inhibitors for multiple metals and alloys including magnesium and its alloys. However, the use of chromates will have to come to the end by 2017 due to EU regulations. Thus, new corrosion inhibitors are needed for replacing the chromates without compromising their high efficiency. Nowadays, chromium-free inhibitor solutions do not provide equivalent inhibiting efficiency.
U.S. Pat. No. 6,569,264 B1 discloses a corrosion inhibitor composition for magnesium or magnesium alloys for use in protective coatings, which contains as an effective component, a phosphate, at least one compound selected from among aromatic carboxylic acids (e.g. toluic acid) or salts thereof and a pyrazole or triazole (such as 1,2,3-triazoles or 1,2,4-triazoles). Published European patent application 1 683 894 A1 discloses the use of 1,2,4-triazoles or pyrazole compounds (e.g. 3,5-dimethylpyrazole) as useful corrosion inhibitors for magnesium and magnesium alloys, which could be incorporated in protective coatings.
Co-pending European patent application 15189674.3 discloses that noble impurities like, iron, copper and nickel, although being the sites of cathodic reaction, get detached from the corroding magnesium by undermining mechanisms and dissolve by forming Fe(II), Fe(III), Cu(I), Cu(II) and Ni(II) ions. This enlarges the area of cathodic activity and accelerates corrosion. Subsequently, these ions are being reduced and re-deposit on the surface of the magnesium or magnesium alloy. Corrosion inhibitors for magnesium or magnesium alloys that prevent re-deposition of the ions are preferably selected from salicylic acid and their derivatives, acid and salts thereof.
However, there is still the need to provide corrosion inhibitors for use in coatings for magnesium or magnesium alloys which have a still improved effectiveness for a wide range of magnesium alloys.